import React, { useEffect, useState } from 'react'

export default function App2() {
    let [opacity, setOpacity] = useState(1)
    useEffect(() => {
        const opacityTimer = setInterval(() => {
            opacity -= 0.1
            if (opacity < 0) {
                opacity = 1
            }
            setOpacity(opacity)
        }, 100);
        return () => {
            clearInterval(opacityTimer)
        }
    }, [])
    return (
        <div>
            <h1 style={{ opacity }}>react太难，不学了</h1>
        </div>
    )
}
